The present invention relates to a sewing machine having an automatic thread tightening device which supplies an upper thread in a suitable length required for forming stitches.
A conventional sewing machine having an automatic thread tightening device forms proper stitches by crossing an upper thread and a lower thread almost at a center of the thichness of a fabric to be sewn. A detecting device of stitching conditions detects fabric feed amounts, needle swinging, fabric thickness and thread thickness. A selecting input device controls thread tension through an upper thread tension member or an upper thread supply device in response to the stitching conditions. The upper thread tension member is in the form of a pair of spring biased disks which frictionally clamp the upper thread. The clamping pressure is controlled in response to the detected stitching conditions. Data corresponding to the detected stitching conditions are processed by a computer which delivers a control signal to the selecting input device to set the clamping pressure at the most optimum level for a particular stitching condition. Arrangements of this kind are known from U.S. Pat. Nos. 4,690,083 and 4,321,307 the entire disclosures of which are incorporated herein by reference. However, such arrangements are very complicated in structure, and encounter difficulties when threads of different materials are used.